Optoelectronic devices generally include light-emitting devices and photovoltaic devices. These devices generally include an active layer sandwiched between two electrodes, sometimes referred to as the front and back electrodes, at least one of which is typically transparent. The active layer typically includes one or more semiconductor materials. In a light-emitting device, e.g., an organic light-emitting diode (OLED) device, a voltage applied between the two electrodes causes a current to flow through the active layer. The current causes the active layer to emit light. In a photovoltaic device, e.g., a solar cell, the active layer absorbs energy from light and converts it to electrical energy which generates a flow of current at some characteristic voltage between the two electrodes.
One way to configure the device electrically has been termed ‘monolithic series interconnection’ and is described in U.S. Pat. No. 7,049,757, and U.S. Pat. No. 7,518,148, both assigned to the General Electric Company. In this configuration, shown schematically in FIG. 1A, device 100 is composed of two individual or pixels 110 and 120 disposed on substrate 130 and are electrically connected in series. Device 100 is depicted as having two pixels, but an arbitrary number of pixels may be connected in series. Pixel 110 consists of anode 112 and cathode 114 with electroactive layer 116 between; likewise, pixel 120 consists of anode 122 and cathode 124 with electroactive layer 126 between.
The series interconnection in device 100 is made by overlapping cathode 114 and anode 112, forming interconnection zone 140. Each pixel operates at a nominal voltage V and current i. The end to end applied voltage is therefore 2V and the applied current is i. The amount of current required to illuminate each pixel is proportional to its size. For a larger pixel, the higher current increases the resistive loss as the current spreads across the correspondingly larger electrodes. The resistive loss is realized as a voltage drop and is calculated as V=iR. Thus a large pixel with a higher required current also exhibits a greater voltage drop across the electrode and results in non-uniform brightness across the pixel. This series design reduces brightness variation by using smaller pixels connected in series so that the resistance loss is less. If the pixels are too large, then the voltage drop results in non-uniform current density through the emissive layers and therefore brightness variation within the pixel. Typically the transparent electrode is the limiting factor because its sheet resistivity is larger than an opaque (possibly metal) electrode.
FIG. 1B shows details of the structure of OLED 100 that are relevant to fabrication. The device is fabricated by depositing and patterning the various layers in a sequential process. In one example, a continuous layer of indium tin oxide (ITO) supported on a glass or plastic substrate 130 is scribed at 101 using a mechanical, laser or chemical etching process to form patterned anodes 112 and 122. Alternately, the ITO may be deposited through a mask to form the pattern. Electroactive layers, typically including, for example, a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer, are deposited, each as a continuous layer, and then selectively removed by solvent wiping or another removal process to form scribe 102. Alternately, the electroactive layers may be deposited in the required pattern using a printing process such as inkjet printing, or a selective coating process. Finally the cathode 114/124 is deposited by, for example, evaporating a metal layer through a mask, forming scribe 103. The entire process requires precise registration and alignment at each step to minimize the dark area that extends from scribe 101 to scribe 103 in areas lacking an anode layer, an electroactive layer or a cathode layer.
The device is energized by providing external connections to the cathode (V0) and the anode (V2). The voltage V1=(V0+V2)/2. Region 109 between scribe 101 and scribe 103 is not illuminated because the anode and cathode are at the same voltage (V1) in this region, and light is emitted only as shown by arrows 117 and 127. The scribe lines and resulting dark areas that appear as continuous lines interrupt the otherwise uniform light output. The dark area may be reduced by minimizing the width of scribes and the spacing of the scribes, but cannot be completely eliminated. Therefore there is a need to find alternate methods to construct a device to increase the size of the pixel and reduce the dark area.
The key factor that limits pixel size is sheet resistivity of the ITO anode and the resulting voltage drop. For example, a metal cathode composed of aluminum is relatively conductive, having sheet resistivity <<10 ohms/square, while an ITO anode is much less conductive, having resistivity >>10 ohms/square. Thus, the cathode is essentially at a uniform electrical potential that is equal to the applied voltage because the iR losses are small because R is small. However, the relatively higher sheet resistance of the ITO causes a larger iR loss and corresponding voltage non-uniformity across the anode. Thus the voltage difference between the cathode and the anode varies with location and the brightness of the pixel is therefore non-uniform. In order to overcome the limitation of ITO (or other transparent conductor) resistivity, it may possible to augment the conductivity by making the layer thicker (but therefore less transparent) or by adding a thin metal layer or metal grid underneath the ITO, but also at the expense of making the layer less transparent. Given the fact the pixel size is limited, the width of dark lines can be reduced with tighter manufacturing tolerances, but there is a lower limit to the spacing of the pixels and the dark lines remain visible.
A further limitation of the series design is that total number of pixels may be limited by the maximum acceptable externally applied voltage for safety reasons. That is, there is a limit to how many series connections can be made before the external voltage that must be applied would exceed product limitations. For example, 50V might be suitable for connecting 10 pixels in series, but 500V for 100 pixels would typically not be suitable for a consumer product.
Therefore, a different structure for an optoelectronic device would be desirable in order to reduce processing costs, reduce dark areas for OLEDs and non-absorbing areas for photovoltaic (PV) devices, and, especially, to allow for large area pixels.